In both low pressure and high pressure gas discharge devices and lamps, low work function thermionically emitting electrodes are needed for efficient operation. In a low pressure device, a low work function may be achieved by applying a coating of an emitter such as barium oxide BaO to the surface of the electrode. Since the diffused arc terminus readily obtained in a low pressure device is not destructive of such an oxide coating, long life may be obtained. But in high intensity discharge lamps wherein the pressure is upwards of one atmosphere, the arc concentrates to a high current density and forms what is generally referred to as a hot spot which is destructive to ordinary oxide coatings. For this reason most commercial high intensity discharge lamps utilize electrodes comprising a rod or shank around which is wound a tungsten coil structure. The emission material is held as a polycrystalline powder in the interstices between turns by an overwind coil and is expected to provide fractional monolayer coverage of the tip of the shank projecting beyond the coil where it is hoped the arc terminus will attach.
In high intensity discharge lamps utilizing tungsten shank plus overwind type electrodes, the method by which the emission material migrates to the tip is not well defined. Nor is it always very effective because the arc terminus in lamps with shank plus overwind electrodes is frequently observed to attach to the coil during the cathode half cycle and to the shank tip during the anode half cycle. This "split spot" mode is destructive to the electrode because it deprives the electron-emitting cathode region of most of the heat supplied during the anode half cycle. This makes it necessary for the required heat to be produced during the cathode half cycle alone, and such requires substantially increased cathode fall voltage and an increased function of current carried by positive ions. The "split spot" mode causes loss of metal from the electrode to the wall and also tends to cause sputtering of emission material from its intended reservoir and location in the coil.